The present invention relates to the field of depolymerization and purification of recyclable polyethylene terephthalate. More specifically, the present invention relates to ammoniolysis depolymerization of polyethylene terephthalate to form terephthalic acid and ethylene glycol, which may be used as starting materials in the production of a recycled polyethylene terephthalate product.
It is increasingly important to provide economically feasible processes for recycling waste. One such waste is recyclable polyethylene terephthalate (PET). xe2x80x9cRecyclable PETxe2x80x9d includes PET that is contaminated with a material present on the polymer surface or diffused into the polymer, PET that is copolymerized with modifying agents, PET that is formed into an article containing layers or coatings of other materials, and scrap PET produced during various molding, film, and fiber production processes. Much recyclable PET is contaminated to the extent that it must be depolymerized and purified prior to being recycled for use as food packaging. Common contaminants include colorants and dyes, opacifiers, polymerization catalyst metals, polymer modifiers, barrier resins, and oxygen absorbers. The predominant source of recyclable PET is discarded PET soft drink bottles. Scrap PET fiber, scrap PET film, and poor quality PET polymer are also major sources of recyclable PET.
PET is commercially manufactured from ethylene glycol (EG) and either dimethyl terephthalate (DMT) or terephthalic acid (TPA). The DMT route was the first commercialized, but for economic reasons, all modern PET manufacturing plants use the TPA route. It is known that methanol may be used to depolymerize PET to form DMT. As noted above, the DMT must be converted either to TPA by hydrolysis or to polyester monomer by reaction with EG before it can be used in a TPA-based PET production process. The cost of these conversions and the necessary purification steps is high. In light of the above, it would be desirable to provide a depolymerization and purification process wherein the TPA and ethylene glycol monomers for use in the production of a new PET product are directly formed during depolymerization of recyclable PET.
TPA can be made from recyclable PET by neutral hydrolysis using water, but this is not economically attractive because of the high temperature and pressure required and because of the difficulty of purifying the resulting TPA.
TPA can be made from recyclable PET by saponification of PET using sodium or potassium hydroxide. The resulting sodium or potassium terephthalate salt is converted to TPA using a strong mineral acid, such as sulfuric acid. This method is deleterious in that the sodium or potassium hydroxide and the required mineral acid are expensive. Additionally, the salt byproduct formed has little economic value. Accordingly, such known saponification processes for the production of EG and TPA from PET are economically unattractive.
U.S. Pat. No. 4,542,239 discloses a process for recovering terephthalic acid from waste polyethylene terephthalate by ammoniolysis. PET is reacted with ammonium hydroxide to form diammonium terephthalate and ethylene glycol. The diammonium terephthalate is converted to TPA using sulfuric acid, thereby producing an ammonium sulfate salt. While the ammonia may be recovered from the salt for reuse in the process by the addition of a base, such as calcium hydroxide, the process produces a nearly valueless gypsum byproduct and consumes valuable sulfuric acid and calcium hydroxide.
Accordingly, there is a need for a new process for directly forming TPA from recyclable PET.
The present invention is a process of forming reactants useful in the manufacture of polyethylene terephthalate that comprises the steps of: (a) treating recyclable PET with ammonium hydroxide whereby a mixture of ammonium terephthalate and EG is formed, and (b) heating the ammonium terephthalate so formed at a temperature of from about 225xc2x0 to about 300xc2x0 C. whereby TPA and ammonia are formed. The present process preferably further comprises a step of polymerizing the EG and the TPA so formed under esterification and polycondensation conditions whereby a PET product is formed.